Apart from the aforementioned consensus protocols, there is a separate class of consensus protocols for blockchains where the ledger is modeled as a directed acyclic graph (DAG). These blockchains either have blocks as vertices in the DAG or have transactions as vertices in the DAG. In block-based DAG, each block is hash linked to multiple parent blocks instead of just a single one as in the case of traditional blockchains such as Bitcoin and Ethereum. The selection of parent blocks for the same is a significant issue for every new block. Unlike serially linked blockchains, multiple blocks get added at different points, and therefore, the problem of conflicting transactions may arise. There are two consensus protocols proposed based on block-based DAG ledger, PHANTOM [1]} and SPECTRE [2]}. Both follow PoW based block proposal and validation. The transaction-based DAG blockchain is adopted through its consensus protocol Tangle [3]}. The tangle protocol is used by IOTA, a cryptocurrency-based distributed ledger designed for IoT devices. In IOTA, a node can add a new transaction after verifying at least two existing transactions and attaching its transaction to these two. The attachment is made by including the hash values of the selected transactions for validation and solving the PoW puzzle to broadcast along with the transaction. Since cryptocurrency is involved, the malicious entities can carry out feature-based exploitation of IOTA. The IOTA tangle provides protection against Denial of service and spam attacks, but not against social engineering based attacks.
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